The present invention relates to a maintenance technology of an incore piping section of a reactor such as a boiling water reactor or the like, and in particular, to an incore piping section maintenance system of a reactor, which performs preventive repair and preventive maintenance of weld (welded or to be welded) zones or portions in an incore piping section located in a reactor pressure vessel.
A boiling water reactor such as a light water reactor is constructed as shown in a longitudinal cross-sectional view of FIG. 7. A reactor core 2 is installed in a reactor pressure vessel 1, and the reactor core 2 is immersed in a coolant 3. Further, the reactor core 2 is constructed in a manner that a plurality of fuel assemblies (not shown) and control rods are arranged in a cylindrical core shroud 4.
A reactor water (coolant) 3 in the reactor pressure vessel 1 flows upward through the core 2 from a core lower plenum 9. The coolant 3 receives a nuclear reaction energy when flowing upward through the core 2, and then, its temperature and pressure rise up, and thus, becomes a two-phase flow state of water and steam (vapor). The coolant 3, which is in a gas-liquid two-phase flow state, flows into a steam separator 5 located above the reactor core 2, and then, is separated into water and steam by means of the steam separator 5. A steam thus gas-liquid separated is introduced into a steam desiccator or drier 6 located above the steam separator 5, and then, is dried here so as to become a dry steam. The dry steam is supplied as a main steam to a steam turbine (not shown) via a main steam pipe (tube) 7 connected to the reactor pressure vessel 1, and then, is used for power generation.
On the other hand, a water thus gas-liquid separated is guided to a truss or sleeve-like downcomer portion 8 between the reactor core 2 and the reactor pressure vessel 1, and then, flows downward through the downcomer portion 8, and thus, is guided to a core lower plenum 9. Further, in the downcomer portion 8, an outer periphery of the core shroud 4 is provided with a plurality of jet pumps 10 at equal intervals.
Meanwhile, the core lower plenum 9 below the reactor core 2 is provided with a control rod guide pipe 11, and a control rod driving mechanism 12 is located below the control rod guide pipe 11. The control rod driving mechanism 12 carries out a control for inserting and pulling a control rod into and out of the reactor core 2 through the control rod guide pipe 11, and thus, performs a power control of reactor.
Moreover, two reactor re-circulation systems including a reactor re-circulation pump (not shown) are located outside the reactor pressure vessel 1. When the re-circulation pump of the reactor re-circulation system is operated, a coolant in the reactor pressure vessel 1 passes through a reactor re-circulation system (not shown) from a cooler re-circulation water outlet nozzle 12, and then, is returned into the reactor pressure vessel 1, and thus, is guided to the jet pump 10 via the re-circulation water inlet nozzle 13. The jet pump 10 sucks its surrounding coolant, and then, supplies it into the core lower plenum 9. More specifically, by a driving water supplied from the reactor recycle pump to the jet pump 10, the jet pump 10 forcibly circulates the coolant 3 in the reactor core 2 via the core lower plenum 9.
On the other hand, the reactor pressure vessel 1 is provided with a core spray system 15 which constitutes an emergency cooling system of a reactor. The core spray system 15 has a piping arrangement as shown in FIG. 5 and FIG. 6. FIG. 6 is a perspective view showing a state that the core spray system 15 is located in the reactor pressure vessel.
As shown in FIG. 5, the core spray system 15 extends into the core shroud 4 from the outside of the reactor pressure vessel 1 penetrating through the reactor pressure vessel (RPV) 1 and the core shroud and includes a core spray system pipe for introducing a spray water into the core shroud 4. The core spray system pipe is a piping part for connecting the RPV 1 and the core shroud 4 in the RPV 1.
Moreover, a pipeline of the core spray system 15 is arranged as shown in FIG. 6. In the core spray system 15, an incore branch part 16 is connected to one end of the core spray system pipeline after penetrating through the RPV 1. A semi-circular pipe 17 is formed in a manner of extending from the incore branch part 16 like a semicircular arc and branching right and left. Each end portion of the semi-circular pipe 17 branching right and left is formed at a position separating by an angle of about 180.degree. along an inner wall of the RPV 1. The semi-circular pipe 17 is connected with a vertical pipe 18 which extends downward from each end portion thereof. A lower end of the vertical pipe 18 constitutes the other end of the core spray system pipeline. A lower end of each vertical pipe 18 is connected via a sleeve 20 to a riser pipe 19 which rises up from the core shroud 4, and thus, a core spray system pipeline is constructed. The core spray system pipeline functions as a reactor emergency cooling system into which a cooling water for cooling the core is supplied in a reactor emergency shutdown. When the emergency cooling system is operated, a fluid vibration, thermal deformation and the like are generated in the core spray system pipeline.
For this reason, the core spray system pipeline is used under severe circumstances as compared with other equipments, and as a result, a great load is applied to each member of the core spray pipeline, and as the case may be, a great stress is applied to the core spray pipe.
Some early nuclear power plants have been operated for more than twenty years, and hence, stable operation for aged plants makes it more vitally important to implement the preventive maintenance of a reactor pressure vessel and internal elements of the early plants which were made of high carbon stainless steel susceptible to Stress Corrosion Cracking (SCC). As mentioned hereinlater, the SCC is caused by the combination of three factors of Material, Stress and Environment, and it is important to get rid of one of three factors for the preventive maintenance.
In the event that an excessive load is applied to the core spray pipe of the core spray system 15 due to any factors, or an inner surface of the core spray pipe rusts away, there is the possibility that a crack or the like is generated in the pipe due to the rust.
Furthermore, because an austenitic stainless steel pipe is mainly used as a material for the core spray pipe, if the following three factors, that is, Stress, Corrosion Environment and Material (generation of chromium deficiency layer) are realized, the Stress Corrosion Cracking (SCC) is generated, and for this reason, it is anticipated that the core spray pipe is damaged.
This stress corrosion cracking phenomenon does not happen if any one of the three factors, mentioned above, lacks. In order to prevent this stress corrosion cracking, there is a need of making various measures so that the aforesaid three factors are not established. Moreover, in the case where a rush and crack is generated in a surface of the core spray pipe due to any factors, when these rush and crack have left, the crack is progressing, and as a result, there may be the case where a crack is generated in the core spray pipe. Thus, when the core spray system 15, which functions as an emergency cooling system of a reactor, becomes a state as described above, it is anticipated that a harmful influence is given to other equipments included in the core, thus being not preferable.
Furthermore, recently, a laser de-sensitization treatment (LDT) technology has mainly been developed for the preventive maintenance of the thin pipe and plate. A high power laser beam produces a molten layer and solution heat treated layer and can change the sensitized surface of a stainless steel to be de-sensitized.
The LDT is a treatment for suppressing a sensitivity (de-sensitization) of an Intergranular Stress Corrosion Cracking (IGSCC) by the steps of irradiating with laser beams a surface of a stainless steel sensitized by an influence of welding heat or the like and forming a solution heat treatment layer and a molten coagulation layer.
That is, FIG. 8 shows relationship among the above mentioned three factors such as Material, Stress and Environment for improving the SCC proof property in view of the de-sensitization treatment, and as shown in FIG. 9, when a YAG laser beam of high energy density passing through an optical fiber, for example, is irradiated on a laser execution portion through optical means such as mirror or lens, the portion subjected to the laser execution is rapidly heated, a Cr carbide is decomposed and, hence, a Cr-lacking layer near a grain boundary is lost. After the laser beam has passed, the laser execution portion is rapidly cooled and the surface thereof is de-sensitized. By continuously performing such de-sensitization treatment to the surface contacting the solution, the solution heat treatment layer and the molten coagulation layer are formed.